Rangefinders

ABSTRACT

In a laser rangefinder a gating pulse is generated after each laser pulse emission and the time-positions of successive gating pulses in the intervals between laser pulses are progressively shifted and correspond to progressively changing ranges for reflected light pulses. When a signal corresponding to a reflected light pulse falls within the gating pulse period, the gated output is integrated and when the integrated value exceeds a threshold value the progressive alteration of the time positions of the gating pulses is halted.

United States Patent 1191 1111 3,723,002 Everest et al. 1 1 Mar. 27,1973 [54] RANGEFINDERS 3,630,616 12 1971 Everest ..343 7.3

[75] Inventors: Frank G. Everest, Stevenage;

R d L Alb giggleswade, Primary Examiner--Benjamin A. Borchelt both ofEngland Assistant ExaminerS. C. Buczinski [73] Assignee: BritishAircraft I Corporation, Atmmey Rlchard Sughrue at Limited, London,England [22] Filed: Dec. 7, 1971 [21] Appl. No.: 205,596

[57] ABSTRACT In a laser rangefinder a gating pulse is generated aftereach laser pulse emission and the time-positions of successive gatingpulses in the intervals between laser [30] Foreign Application PriorityData pulses are progressively shifted and correspond to Dec. 10,1970Great Britain ..58,747/70 pmgressively changing ranges for reflectedlight ses. When a signal corresponding to a reflected light 52 U.S. c1.,.356/5, 356/4, 343/73 Pulse falls Within the gating Pulse Period, thegated [51] Int. Cl. ..G0lc 3/08 Output is integrated and when integratedValue [58] Field of Search ..356/4, 5; 34317.3 Ceeds a threshold valuethe progressive alteration of the time positions of the gating pulses ishalted. [56] References Cited 6 Claims, 1 Drawing Figure UNITED STATESPATENTS 3,648,286 3/1972 Schoneborn ..343/7.3

LASER our/ 07 o/s uy fig 5 M552 /5 i 24 /5 1 05756701 1 1 Am a 1 f z .1PRF-AMP amp/1701a I 8 l 1 1 tom/um 1 l l 1 i l 1 PHASE 1 6NRA70R l 26 2328 i we? THRESHOLD COMPARAZUR RANGEFINDERS This invention is concernedwith rangefinders of the kind in which a laser emits a succession oflight pulses and a detector detects the receipt of reflected laser lightand generates a corresponding electric signal. The interval between atransmitted laser pulse and the received reflection represents the rangeof the reflecting object. In systems using pulsed lasers capable of highpulse repeat frequency (such as gallium arsenide lasers, for example)there is a limit to both the peak output power and to the mean outputpower of the laser. The low power of a reflected pulse derived from sucha system and the low signal/noise ratio makes detection difficult.

However, there is no limit to the pulse repeat frequency that can beused with such a laser, provided that the mean power limit of the laseris not exceeded and the present invention relies on pulse integration toimprove the signal/noise ratio in a system employing a laser of highpulse repeat frequency.

A rangefinding system according to the present invention includesscanning pulse-generating means for generating a series of electricpulses, one in each interval between laser pulses, the scanningpulse-generating means including timing means for progressively alteringthe time-position of the generated pulse within the interval betweensuccessive laser pulses so that the moments of occurrence of thegenerated pulses in the intervals correspond to progressively changingranges for reflected light pulses, gating means responsive to thegenerated pulses to pass a signal representative of a reflected laserpulse only when a generated pulse is present, means integrating theoutput of the gating means, and means connected with the timing meansand with the integrating means and responsive when the integrated valueexceeds a threshold value to stop the progressive alteration of the timeposition of the generated pulse in the sense in which the saidalteration was progressing.

In the preferred form of rangeflnder embodying the present invention aD.C. restoring circuit is connected between the output of the detectorand the gating means The effect of such'a circuit, if the time-width ofthe gate is suitably chosen in relation to the duration of the reflectedpulse, is to ensure that as the gating pulse moves across the signalrepresenting the reflection, the output of the gate is first of onepolarity and then of the other. Consequently, the gate output applies apositive polarity to thethreshold detector over only a narrow range ofgate positions in relation to the reflection signal and enhancedpositional accuracy is thereby obtained.

The means for progressively altering the time-position of the generatedpulses may consist of a first ramp generator which generates a rampwaveform following each laser pulse and a second and slower rampgenerator generating a ramp waveform which extends over a number ofcycles of the first ramp generator waveform, together with a comparatorcircuit which generates a pulse each time that the levels of the tworamps are the same.

In order that the invention may be better understood, one example ofapparatus embodying the invention will now be described with referenceto the accompanying drawing.

In the drawing, a laser pulse generator It) drives a laser 12 at a highpulse repetition frequency and the resulting short laser pulses aredirected at a target through a lens 14. As each laser pulse istransmitted a synchronized pulse from the generator 10 triggers a rangeramp generator 16 in a scanning pulse generator 15. Thus, for eachtransmitted laser pulse the generator 16 provides a ramp waveform havinga rate of l volt/microsecond, for example. This ramp waveform is appliedto one side ofa comparator 18.

A slow scan generator 20 provides a ramp waveform extending over anumber of cycles of the rampgenerator 16 and this slow ramp waveform isapplied to the other side of the comparator 18. Each time that theinstantaneous level of the fast ramp waveform from generator 16 equalsthe instantaneous value of the slow ramp waveform from the generator 20,the comparator 18 generates an output pulse which triggers a pulsegenerator 22. It will be seen that the pulses from the comparator occurat progressively changing time-positions in the interval betweensuccessive laser pulses and thus correspond to a progressively changingrange. The generator 20 can thus be considered as a range scanner which,in this example, moves slowly downwards from a maximum output to aminimum output, corresponding to scanning from maximum range to minimumrange.

The pulse from generator 22 is applied to a gating and D.C. restoringcircuit 23 which includes a signal gating switch S1 and a signalstarting switch S2. It will be appreciated that although these switchesare represented diagrammatically as simple contact switches in thedrawing, in practice electronic switching circuits of conventionaldesign are used. In the condition shown in the diagram, switch S2conducts the output of capacitor C2 to earth and switch S1 is in itsnon-conducting condition. When a pulse is received from generator 22switch S1 becomes conductive and simultaneously switch S2 removes theearthing circuit from the output of capacitor C2. Thus, switch S2 hasthe effect of inserting a D.C. component at the output of capacitor C2in the intervals between gating pulses. A detector and preamplifiercircuit 24 receives light reflected from any reflecting object in thepath of the laser beam and applies a corresponding electric signal tocapacitor C2. If while the switch S1 is in its signalpassing conditionthere is a reflection signal at the output of capacitor C2 (that is tosay if the light-reflecting object is at the range represented at thatinstant by the output level of the slow scan generator) this signal ispassed through the switch S1 to an integrator consisting of a capacitorC1 and a resistor R1. Between gating pulses from the generator 22, theswitches S1 and S2 revert to their original conditions and the signal oncapacitor C1 is stored. The time constant of the circuit Rl-Cl is chosento obtain the performance required from the apparatus. Typically, itwould represent a period of several hundred gating pulses. There mightbe several thousand pulses in an uninterrupted scan from maximum tominimum range.

The gating pulse width is set to be more than half the expected lengthof the received pulse but less than the total length of the receivedpulse. The value of the gating pulse width is, however, not critical.The slow scan rate is such that the reflected pulse from an objectilluminated by the laser beam is likely to be coincident with the pulsesfrom the generator 22 during a number of successive pulses. Thus, thesignals from the preamplifier 24 corresponding to successive reflectionswill be integrated by the capacitor C1.

The first coincidence between the reflectionrepresenting signal frompreamplifier 24 and the rangescan pulse from generator 22 will usuallybe very brief and successive coincidences will progressively increase induration as the range scan pulse moves across" the target reflectionpulse. The charge on capacitor C1 will rise until a threshold voltage,set by a threshold level circuit 26, is reached. A threshold comparator28 detects this condition and applies a signal to a scanning controlcircuit 29. The scanning control circuit contains two switches S3 andS4; these are again illustrated as simple contact switches but inpractice are electronic switching circuits. The signal from thethreshold comparator 28 opens the switch S3 which is in series with aresistor R3 between the input of the slow scan generator and a positiveterminal. The opening of the switch S3 stops the downward scan of theslow scan generator 20. A further resistor R2 connected between theinput of the slow scan generator 20 and a negative terminal ensures thatwhen the switch S3 is open the output of the slow scan generator 20rises slowly, causing the gating pulses to occur at progressively latertime-positions in the intervals between laser pulses until the input tothe threshold comparator falls below the threshold setting. At thispoint, the comparator 28 ceases to hold the switch S3 open and the slowscan generator input is again connected to the positive terminal,thereby bringing the gating pulse back on to the received reflectionpulse.

Thus, the system hunts on to the return pulse, alternately scanning intoit and then slowly moving off again to longer range. The width of thishunting movement" is approximately equal to half the return pulse width.This determines the maximum accuracy which the system can achieve.

The scan limit 30 is used to control the scan generation in the absenceofa return signal. When the value of the output of the slow scangenerator corresponds to the minimum range value, the switch S4 connectsthe slow scan generator input through a resistor R4 to the negativeterminal. The value of resistor R4 is such that the slow scan generatoroutput returns rapidly to a value corresponding to the maximum range, atwhich level the scan limit circuit 30 again opens switch S4 to permit anew scanning cycle to commence. The output of the slow scan generator isalso applied to an output display 32.

We claim:

1. A rangefinder comprising a laser arranged to emit a succession oflight pulses, a detector for detecting the receipt of reflected laserlight and generating a corresponding electric signal, scanningpulse-generating means for generating a series of electric pulses, onein each interval between laser pulses, the scanning pulsegeneratingmeans including timing means for progressively altering thetime-position of the generated pulse within the interval betweensuccessive laser pulses so that the moments of occurrence of thegenerated pulses in the intervals correspond to progressively changingranges for reflected light pulses, gating means responsive to thegenerated pulses to pass a signal representative of a reflected laserpulse only when a generated pulse is present, means integrating theoutput of the gating means, and means connected with the timing meansand with the integrating means and responsive when the integrated valueexceeds a threshold value to stop the progressive alteration of the timeposition of the generated pulse in the sense in which the saidalteration was progressing.

2. A rangefinder in accordance with claim 1, including a D.C. restoringcircuit connected between the output of the detector and the gatingmeans.

3. A rangefinder in accordance with claim 2, in which the D.C. restoringcircuit includes switching means whereby the said circuit is renderedinoperative during the said generated pulses.

4. A rangefinder in accordance with claim 1, in which the scanningpulse-generating means includes second timing means for. progressivelyaltering the time-position of the generated pulse within the intervalbetween successive laser pulses, the second timing means altering thetime-position in a sense opposite to the alteration provided by thefirst timing means, and switching means for rendering the said secondtiming means operative when the integrated value exceeds the thresholdvalue.

5. A rangefinder in accordance with claim 1, in which the scanningpulse-generating means includes a first ramp generator generating a rampwaveform following each laser pulse and a second and slower rampgenerator generating a ramp waveform which extends over a number ofcycles of the first ramp generator waveform, and a comparator circuitconnected to receive ramp signals from the two ramp generators andgenerating a pulse each time that the instantaneous levels of the tworamp waveforms are the same.

6. A rangefinder in accordance with claim 1, in which the laser is agallium arsenide laser.

1. A rangefinder comprising a laser arranged to emit a succession oflight pulses, a detector for detecting the receipt of reflected laserlight and generating a corresponding electric signal, scanningpulse-generating means for generating a series of electric pulses, onein each interval between laser pulses, the scanning pulse-generatingmeans including timing means for progressively altering thetime-position of the generated pulse within the interval betweensuccessive laser pulses so that the moments of occurrence of thegenerated pulses in the intervals correspond to progressively changingranges for reflected light pulses, gating means responsive to thegenerated pulses to pass a signal representative of a reflected laserpulse only when a generated pulse is present, means integrating theoutput of the gating means, and means connected with the timing meansand with the integrating means and responsive when the integrated valueexceeds a threshold value to stop the progressive alteration of the timeposition of the generated pulse in the sense in which the saidalteration was progressing.
 2. A rangefinder in accordance with claim 1,including a D.C. restoring circuit connected between the output of thedetector and the gating means.
 3. A rangefinder in accordance with claim2, in which the D.C. restoring circuit includes switching means wherebythe said circuit is rendered inoperative during the said generatedpulses.
 4. A rangefinder In accordance with claim 1, in which thescanning pulse-generating means includes second timing means forprogressively altering the time-position of the generated pulse withinthe interval between successive laser pulses, the second timing meansaltering the time-position in a sense opposite to the alterationprovided by the first timing means, and switching means for renderingthe said second timing means operative when the integrated value exceedsthe threshold value.
 5. A rangefinder in accordance with claim 1, inwhich the scanning pulse-generating means includes a first rampgenerator generating a ramp waveform following each laser pulse and asecond and slower ramp generator generating a ramp waveform whichextends over a number of cycles of the first ramp generator waveform,and a comparator circuit connected to receive ramp signals from the tworamp generators and generating a pulse each time that the instantaneouslevels of the two ramp waveforms are the same.
 6. A rangefinder inaccordance with claim 1, in which the laser is a gallium arsenide laser.